Pressure sensitive pneumatic switch actuator circuits of the type which actuate a remote electrical switch are well known. One example of such means is the familiar pneumatic circuit used in automobile service stations to signal the presence of an automobile to be serviced. Another example of such a circuit is a traffic counter having a hose which is stretched across a highway to detect the number of cars passing a given point. A third example is the pneumatic bilge pump actuator circuit taught in U.S. Pat. No. 3,091,676, issued to Koster on May 28, 1963.
In each of these known applications, a deformable, fluid filled sensor is deformed when it is contacted by a sensed object (such as water or an automobile). Part of the fluid in the sensor then flows out of the sensor and into a conduit as a pulse of increased pressure. This impulse is transmitted through the conduit to switch actuating means with a closed interior cavity having as one wall a pressure sensitive diaphragm. The pulse transmitted by the conduit into the closed interior cavity displaces the diaphragm outward into contact with the switch lever of an electrical switch which is then actuated. Thus, an impulse is transmitted by non-electrical (hydraulic or pneumatic) means to a remote electric switch to operate the switch.
One advantage of this system is that it keeps electrical switches and wires away from the sensed object. This extends the life of the electrical parts of the system while keeping electricity away from the point of actuation of the safety switch. This system is especially useful in applications where the presence of electrical current is hazardous, as near explosive vapors or highly combustible materials.
The fluid circuit described above does not quickly actuate the electrical switch and release the actuated switch after a time delay. These actuators therefore are not good safety switches. The Koster system described above is not designed to act as a safety switch. In Koster, a flow restrictor 26 is placed in the conduit to restrict the flow of fluid in either direction within the conduit. The stated purpose of the fluid restrictor of Koster is to prevent actuation of the electrical switch by a momentary pressure. Thus, the Koster system is clearly unsuitable for a safety switch actuator which upon even a momentary touch of the sensor must immediately actuate the disabling switch to prevent danger to the sensed object or person.
U.S. Pat. No. 3,548,201, issued to Ive on Aug. 8, 1968, discloses a machine safety switch. In Ive, the sensor is a floor mat or the like containing a conduit through which a constant flow of fluid is maintained. Stepping or otherwise pressing on the mat constricts the conduit, thus reducing the fluid pressure downstream of the mat. Fluid pressure sensing means located downstream of the mat detects this change in fluid pressure and trips a relay which disables the machine. Unfortunately, Ive provides a drastic disabling system in that the relay which is actuated to disable the machine must be reset from a location remote from the sensor. Thus, even a momentary actuation of the safety system shuts down the machine for an indefinite time until the relay can be reached and reset. After the danger is long past, the machine remains disabled. This is particularly a problem when the person who is likely to actuate the sensor is unfamiliar with the machine which is protected, and thus unable to restart the machine.
It is thus an object of the present invention to provide a fluid operated safety switch which is actuated suddenly to disable a machine, and which reenables the machine to operate after a shoft interval of time has passed.
Other objects of the invention will become apparent from the description which follows.